Technical Field
The present invention relates to an electrostatic coating technology, and in more detail, to an electrostatic coater having a rotary atomizing head.
Background Art
It is a well-known fact that electrostatic coaters are widely used. Electrostatic coaters are frequently used in the automobile industry. In the automobile industry, as the coating quality affects the commercial value of an automobile, each manufacturer sets a rigid standard for the coating quality. As such, electrostatic coaters keep evolving in response to strict demands from the automobile industry.
Paint used for coating an automobile includes a solid paint, a clear paint, and a metallic paint. As a metallic paint, there is a so-called pearl paint containing nonmetallic glossy chips such as mica, besides a paint including metallic chips.
As for electrostatic coating using a metallic paint, it is difficult to realize high-level coating quality. Specifically, with a metallic paint, it is known that if a colliding speed of the paint with an automobile body (hereinafter referred to as a “workpiece”), which is an object to be coated, is slow, the finished appearance of the workpiece is dark. It is also known that as the diameter of a paint particle is larger, the finished appearance of the workpiece becomes darker. In order to realize metallic coating having high-level coating quality, a large number of proposals have been made.
An electrostatic coater of a rotary atomization type, disclosed in Patent Literature 1, includes two systems of air ports arranged coaxially with a rotary atomizing head. Air ports of a first system are positioned on a relatively inner peripheral side. Air ports of a second system are positioned on a relatively outer peripheral side. The orientation of the first air ports on the inner peripheral side is parallel to the axis of the rotary atomizing head. The inner shaping air discharged from the first air ports passes through the vicinity of the outer peripheral edge of the rotary atomizing head. The inner shaping air has a higher pressure and a lower flow rate than those of the outer shaping air discharged from the air ports of the second system. By the inner shaping air, atomization of the paint is facilitated. Then, the atomized paint is accelerated toward the workpiece by the outer shaping air, having a lower pressure and a higher flow rate relatively, discharged from the second air ports.
Patent Literature 2 proposes an electrostatic coating method which improves the coating quality and the coating efficiency of a metallic paint. An electrostatic coater of a rotary atomization type to be used in this electrostatic coating method includes one system of air ports. The orientation of the air ports is parallel to the axis of the rotary atomizing head. The shaping air discharged from the air ports passes through the vicinity of the outer peripheral edge of the rotary atomizing head. Patent Literature 2 proposes to control the peripheral velocity of the rotary atomizing head of the coater.
Patent Literature 3 proposes an electrostatic coater of a rotary atomization type capable of improving the coating quality of metallic coating. The electrostatic coater includes a plurality of air ports arranged behind a rotary atomizing head concentrically with the axis of the rotary atomizing head, and shaping air is discharged from the plurality of air ports. The orientation of the air ports, when the coater is viewed laterally, is parallel to the axis of the rotary atomizing head. When the coater is viewed from the front, the air ports are positioned 2 to 3 mm outward from the outer peripheral edge of the rotary atomizing head. The air ports include guide grooves on the tip end side. The shaping air discharged from each of the air ports becomes a jet flow in a state of being twisted in a rotation direction of the rotary atomizing head or a direction opposite thereto, by the guide groove. This means that the shaping air becomes a flow in a state close to a swirling flow, not to say a swirling flow itself. By setting the twisting direction of the shaping air to a direction opposite to the rotation direction of the rotary atomizing head, it is possible to cause the shaping air to strongly collide with the charged paint particles being scattered from the outer peripheral edge of the rotary atomizing head. Thereby, the paint particles can be micronized.
Patent Literature 4 proposes an electrostatic coater of a rotary atomization type by which metallic coating and general coating can be performed with a single coater. That is, Patent Literature 4 proposes a coater which does not deteriorate both the coating quality of metallic coating and the coating quality of general coating using a solid paint or a clear paint other than a metallic paint. The coater disclosed in Patent Literature 4 includes air ports arranged behind a rotary atomizing head, on first and second circumferences coaxial with the rotary atomizing head. A plurality of first air ports arranged on the first circumference of the inner peripheral side discharge first shaping air toward the rear surface of the rotary atomizing head. Second air ports arranged on the second circumference of the outer peripheral side discharge second shaping air toward the outer peripheral edge of the rotary atomizing head.
The orientation of both the first and second air ports is parallel to the axis of the rotary atomizing head when the coater is viewed laterally. The first shaping air directed to the rear surface of the rotary atomizing head is a straight flow. On the other hand, the second shaping air directed to the outer peripheral edge of the rotary atomizing head is a jet flow in a state of being twisted about the axis of the rotary atomizing head. It should be noted that Patent Literature 4 fails to clearly describe whether the second shaping air is twisted in a rotation direction of the rotary atomizing head or in a direction opposite to the rotation direction of the rotary atomizing head.
The first shaping air directed to the rear surface of the rotary atomizing head is used for general coating, that is, coating using a solid paint, for example. Meanwhile, the second shaping air directed to the outer peripheral edge of the rotary atomizing head is used for metallic coating. As such, in the coater of the Patent Literature 4, each of the first shaping air and the second shaping air is used properly, depending on the case of general coating or the case of metallic coating.
Patent Literature 5 proposes an electrostatic coater of a rotary atomization type capable of improving atomization of paint and coating efficiency and also improving the coating quality of metallic coating. The coater disclosed in Patent Literature 5 adopts a configuration in which first shaping air, second shaping air, and third shaping air are directed to the paint, in a particle state, scattered from the outer peripheral edge of the rotary atomizing head. Patent Literature 5 discloses various specific examples. One example will be described below. A coater of an embodiment includes first, second, and third air ports arranged sequentially in a radial direction from the axis of the rotary atomizing head. The first to third air ports are positioned behind the rotary atomizing head.
The first to third air ports are directed to a direction opposite to the rotation direction of the rotary atomizing head, and shaping air discharged from each air port is a jet flow in a state of being twisted in the opposite direction of the rotation direction of the rotary atomizing head. The first and the third air ports, positioned on the innermost periphery and the outermost periphery, are tilted by 30° in the circumferential direction of the rotary atomizing head. The second air ports, at an intermediate position, are tilted by 15° in the circumferential direction of the rotary atomizing head. From the first air ports positioned on the innermost periphery, first shaping air, having a high speed and a low flow rate, is discharged. From the second air ports at an intermediate position, second shaping air, having a high speed and a low flow rate, is discharged. From the third air ports positioned on the outermost periphery, third shaping air, having a high speed and a low flow rate, is discharged. By adjusting the first to third shaping air, the particle diameter of the paint particles, a coating non-volatile (NV) value, an air impact force, and the like are optimized.
Here, a coating non-volatile (NV) value is recognized as an index of evaluating the appearance quality of coating. A coating non-volatile value is defined by the following expression.Coating NV value (%)=(coating film weight after drying/coating film weight at the time of coating)×100
The coating NV value is described in detail in Patent Literature 6. As such, the description thereof is omitted by incorporating Patent Literature 6 herein by reference.